Although it has been established for many years that the lymphoid lineages derived from multipotential stem cells, precursors representing intermediate stages of development between the stem cell and B and T cell restricted precursors are not well defined. We have recently identified a fetal lymphoid precursor with B cell, T cell and macrophage (B/T/MAC) potential that represents at least one intermediate stage of lymphoid development and as such a precursor that defines a novel commitment step within the hematopoietic system. The overall goals of this proposal are to take advantage of these findings to define early commitment step within the lymphoid system. The first aim of the proposal is to further characterize the B/T/MAC precursor with respect to cell surface phenotype, growth requirements and developmental potential, including natural killer (NK) and dendritic cell potential. These experiments will enable us to define the relationship of this precursor to other lymphoid precursors that have been identified and to more precisely position it within the hematopoietic hierarchy. In the second aim, we will define the developmental origin of B/T/MAC precursor, track its fate in embryonic and fetal development. The outcome of these experiments will provide new information on the role of this precursor in fetal lymphopoiesis and in fetal thymic development. The third aim of the proposal is the characterization of the molecular events involved in lymphoid commitment. We will focus on the identification and characterization of genes that have been isolated from a subtractive hybridization between a colony derived from a B/T/MAC precursor and a colony derived from a more restricted B/MAC precursor. The ultimate goal of this aspect of the proposal is to define genes that play a critical role in lymphoid development. Together the information obtained from these studies will provide us with a better understanding of the events involved in the commitment of hematopoietic stem cells to the lymphoid lineages. Defining these events in normal lymphoid development will ultimately provide new insights into molecular and cellular changes that can lead to the earliest stages of diseases such as immunodeficiency and leukemia.